They came from Japan, clinging to wreckage. They made it to the U.S., only to be destroyed. They are crabs, sea stars, and kelp.

In his 40 years as a marine biologist, John Chapman has seen a lot of stuff wash up on a lot of beaches. The dock that washed ashore on Agate Beach, Ore., in early June last year was the most interesting thing he’d come across in a while. For one thing, the 188-ton dock was well over 60 feet long, nearly 20 feet wide, and 7 feet tall. But it teemed with much smaller items, too, Chapman could see when he looked up close—barnacles, mussels, a thick brown fringe of algae that hid any number of things.

The next day, Chapman went back with a pair of colleagues from Oregon State University to sample the assemblage more rigorously. Someone found a plaque on the dock with Japanese writing. An official with the Japanese consulate in Portland soon confirmed that the dock was from Misawa, a fishing port on Honshu Island—one of four identical docks wrested from their moorings during the 2011 earthquake and tsunami and cast out to sea.

The proceedings now had a newer and grimmer urgency. The dock was not only a public safety hazard, but also a biological one. Chapman and his colleagues surveyed like mad, scraping off specimens, bagging them for later identification. After two hours they had more samples than they could possibly process. Oregon state officials asked what they should do with everything that was left. Chapman didn’t hesitate. Kill it, he told them. Kill it all.

No one knows exactly how much marine debris was generated by the tsunami that devastated northern Japan. The Japanese government’s estimate, which has more or less entered the official record, is that 5 million metric tons were washed into the sea. Of that, 70 percent is thought to have sunk near the Japanese coast; the remaining 30 percent—roughly 1.5 million metric tons—was slung by the powerful Kuroshio Current across the Pacific Ocean.

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“We knew the stuff was coming,” Chapman says, “but it showed up a lot sooner than we thought it would.” In April 2012 the Coast Guard fired upon and sank the ghost fishing vessel Ryou-Un Maru, drifting and derelict in the Gulf of Alaska. In Oregon the dock from Misawa, which Chapman would take to calling Misawa 1, had drifted more than 4,300 miles across the Pacific in just 451 days.

Its haste would be the first of several surprises. Chapman and his colleagues expected that the creatures attached to the dock would be ocean-going organisms that the structure had picked up on its journey through the open sea. But the dock also supported a host of species that could only be from Japan. “It’s hard to overstate how surprising that was,” Chapman says. Convention had it that maybe, at most, one or two of those species should have survived the harsh pelagic environment. But here was a community that was largely intact, with sponges and mussels and anemones and urchins, all of which were known to live only in nearshore environments. It wasn’t a matter of a few hardy individuals clinging to life, either: In some cases, Chapman could see two or more generations of an organism. More alarming still was that at least two species—a sea star and a type of kelp—were on an international watch list of the 100 worst invaders.

“We had a very substantial edifice of information which said that was not supposed to happen,” Chapman says. So it was with particular interest that he rushed up to Olympic National Park the following December, when another dock (Misawa 3) washed ashore after 570 days at sea. Would the community it supported be similar to that of the first dock?

Chapman would have to wait to find out. Unlike Misawa 1, Misawa 3 wasn’t parked on a nice flat beach. Instead, it was tucked in a cove, wedged on sharp rocks, access to it hazardous. For several days, while the seas were high, Chapman and park scientists could only watch as waves pounded the dock. When the group was finally able to reach it, large patches had been cleared, the biota beaten off by the surf. But still they saw to their surprise that the remaining community was made up of different species than Misawa 1’s.

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Life is always on the move. A new species establishes in San Francisco Bay about every 14 weeks; even more modest Yaquina Bay, Ore., which has significantly less ship traffic, gets a new member every year. Species stow away in ballast water, or attach themselves to vessel bottoms, reliably thwarting quarantine efforts. Biological communities everywhere are constantly being poked, prodded, and tested, as new species look to take advantage of even the slightest openings and opportunities. Species that might seem innocuous, like a humble shore crab barely an inch and a half wide, can have significant impacts on the ecosystems that receive them.

But the Misawa docks, and the tsunami debris more generally, are something different. They are, for Chapman, objects of ambivalence. On the one hand they make him nervous, loaded as they are with potential and known invaders; and ecologists have a bad track record of predicting what species will invade where, so of course it is right and proper that their hangers-on were destroyed as quickly and thoroughly as possible. But the docks are also exciting. They approximate a phenomenon that has been happening for millions of years, one that most scientists never expected to witness in their lifetimes. They blur the conceptual line between what biologists call invasion, and colonization.

“Ocean-scale rafting is a sort of fundamental tenant of marine biogeography,” says Jim Carlton, a biologist at Williams College who has examined the docks with Chapman, and is working with a group of scientists to study tsunami debris. Rafting, for instance, is how Hawaii has its own species of land snails, at least for now. It is how Madagascar has lemurs. (It may also be how cholera eventually reaches the U.S.) But such events are rare; species need to establish only every few hundred thousand years to account for the biodiversity seen on island chains. “I personally have watched the West Coast for 40 years looking exactly for this, for something that crossed the ocean with something arriving alive from the other side,” Carlton says.

The Misawa docks are exactly that sort of event. Even as Misawa 1 and Misawa 3 were blowtorched and scraped clean as quickly as possible, most of the organisms on them got away. On Misawa 1, Chapman thinks more than 60 percent of everything escaped; on Misawa 3, isolated and battered for days before anyone could reach it, the percentage of escapees is sure to be higher. Not all of them will survive, of course. Sometimes, though, all it takes is one.

The docks, therefore, could be thought of as the largest experiment in invasion ecology ever run. What happens when you take four large durable structures, all from the same place (and so with the same species on them), and set them adrift? How long will they take to reach the other side of the ocean? What will survive? What will they pick up? And what if they don’t make the crossing at the same time? What will the resulting communities look like?

Chapman, Carlton, and a host of other invasion ecologists are getting their answers now, and will be for the next several years. Less than 1 percent of that anticipated 1.5 million metric tons has arrived. What remains will show up in waves, depending both on prevailing winds, which drive downwelling and upwelling on the West Coast (i.e., pushing debris towards shore, or drawing it away), as well as the properties of the debris itself. This year most of what has washed up has been smaller, more buoyant, and also more anonymous. At one point a lot of lumber turned up on the beach. Chapman could walk along Agate Beach and see, about every 100 meters or so, a large piece of treated timber that he knew was from Japan. But who really looks that closely at logs?

And so Chapman has an eye out for larger things. Last September, between the landings of Misawa 1 and Misawa 3, he learned that a second dock (Misawa 2) had been spotted off the coast of Hawaii, 30 miles from Maui. It would swing within 15 miles of Molokai before observers lost sight of it as the currents carried it back out to the open ocean. The fourth dock, Misawa 4, has yet to be seen. They are out there somewhere, among a debris field that, according to NOAA estimates, is now spread out across an area roughly three times the size of the United States. Caught in the great gyres of the Pacific, each dock bides its time with its own patient passengers, all of them in search of a new home.

Eric Wagner writes about science and the environment from his home in Seattle, where he lives with his wife and daughter. His writing has appeared in Smithsonian, Orion, and High Country News, among other places.